Music Man 410-65 Service.

[loudthud]

What is the benefit of using the scope over playing/ listening/ tweaking trimpot as I'm only able to do (without a scope) ?

Using an oscilloscope is the difference between knowing you have it right (especially if you have replaced the JFET) and ...

Got the trem sorted too.. turned out it needed another 1/2 trimpot rotation to find another small area on the trimpot ( dead between two 'areas' of the dial where the effect worked, gave me impression Id done all i could).

and ...

So mine is only useable clean. If this clip's -65 amp can do it, so could mine. But what would be hampering mine I wonder?

Without a scope, you're just guessing in the dark.

[Sea Chief]

Using an oscilloscope is the difference between knowing you have it right (especially if you have replaced the JFET) and ...



and ...



Without a scope, you're just guessing in the dark.

Hi Lurch. Ok so I think you mean I cant get it correct without a scope.

It's still not right I know this much (intensity is the same at 3 as it is max, which seems to be pretty strong, but something still isn't feeling right... plus the squelchy distortion feint trem sound used when the preamp vol is maxed).

Also it is twice as strong in the hi mode as lo power mode: but not sure if this is to be expected.

Did someone mention, on one thread, I could conceivably use an ipad with some sort of something, as an oscilloscope? Thanks, SC

[Sea Chief]

Chaps, Ive got rather a high voltage figure, at my standby switch: 390v firstly, settling down after 5 mins it seemed, to 377v.

Which = 780v, 752v B+... yikes.

I think that's getting a bit close to my caps max 450v isn't it? (Or is this again divided by two, as Ive got two sets of filter caps in series-?).

Thanks, Sc

[Sea Chief]

Half B+ can be measured at the closed standby switch.

HH I wonder if you could comment: Ive done as you suggest... & get a very high 377v ( 390v first few mins) at the standby switch.

I should be measuring around 362v.

Would the idea now be to increase the bias a tiny bit to bring this plate voltage down a bit? (25mA presently, meauring 0.5v across the 10r cathode resistor on one 6ca7 tube).

[DrGonz78]

Chaps, Ive got rather a high voltage figure, at my standby switch: 390v firstly, settling down after 5 mins it seemed, to 377v.

Which = 780v, 752v B+... yikes.

I think that's getting a bit close to my caps max 450v isn't it? (Or is this again divided by two, as Ive got two sets of filter caps in series-?).

Thanks, Sc

When you have two electrolytic capacitors in series the voltage ratings are summed together. Essentially two 450v caps in series will give a 900v overall voltage rating in this instance.

[Helmholtz]

HH I wonder if you could comment: Ive done as you suggest... & get a very high 377v ( 390v first few mins) at the standby switch.

I should be measuring around 362v.

Difference is only 4%, so negligible.

Would the idea now be to increase the bias a tiny bit to bring this plate voltage down a bit? (25mA presently, meauring 0.5v across the 10r cathode resistor on one 6ca7 tube).

No, because that is likely to increase plate dissipation even more.

EL34s can be used up to 800V plate voltage as long as screen voltage is below 425V.

[Sea Chief]

Difference is only 4%, so negligible.




No, because that is likely to increase plate dissipation even more.

EL34s can be used up to 800V plate voltage as long as screen voltage is below 425V.

Ah yes my maths so poor I thought the difference was greater than it is.

So would decreasing the 0.5v figure slightly at the cathode resistor bring the plate voltage down a bit? Or wouldyou not even bother? They are the original tubes you see.. & the figure was 0.36v at this resistor before i tweaked it up. I don't want to treat them hard really.

thanks SC

[Helmholtz]

So would decreasing the 0.5v figure slightly at the cathode resistor bring the plate voltage down a bit? Or wouldyou not even bother? They are the original tubes you see.. & the figure was 0.36v at this resistor before i tweaked it up. I don't want to treat them hard really.

Lowering idle/cathode voltage will increase B+ and plate voltage.
I'd leave it as it is.

[g1]

Never use bias to try to manipulate plate voltage.
The high voltage may be due to the line voltage being higher than it was when these amps were built. You still have adequate safety margin so no need to worry about it.

[Sea Chief]

Never use bias to try to manipulate plate voltage.
The high voltage may be due to the line voltage being higher than it was when these amps were built. You still have adequate safety margin so no need to worry about it.

Aha ok thanks g1. I'll remember that advice.

Line voltage being our UK 240v? I read in the music man forum, someone worried about the zener diodes ( if i recall) being able to cope, these 2275-65 amps apparantly mfr'd for 220v ? If that is true for 1976. I think maybe were at 230v. so plate voltage difference seems very plausiblly close maybe.

Should I be concerned about my zeners?

[Sea Chief]

Never use bias to try to manipulate plate voltage.
The high voltage may be due to the line voltage being higher than it was when these amps were built. You still have adequate safety margin so no need to worry about it.

G1 isn't my 780v it showed ( by way of measuring 390v at standby switch ) fo a full 5 minutes before creeping down / settling at 762v.. cause for alarm if the tubes might be able to work 'up to 800v' though ? 20v off the top top max figure, both tubes for 5 minutes ? (or even one higher than the other if not matched as they once were which seems likely if from 1976).

Is there no way I can get this plate voltage down a bit? Can I calculate what it was before I tweaked the bias pot, when I measured 0.36v across the 6ca7 10r cathode resistor? Im a bit confused if this plate voltage would have been -higher- or likely -lower- with a lower tube current.

thx SC

[g1]

Change in plate voltage is a byproduct of idle current. The more current the tubes conduct, the more 'work' the power transformer is doing. That makes the voltage sag.
Like a garden hose pressure will drop as you increase the water flow.
When you had .36V across the cathode resistor, that worked out to 18mA per tube, lower current than what you have now. So the voltage would have been higher. You can't calculate what it would have been, just set it back to that setting and measure again at the standby switch.
Now consider that the amp will have been working fine for many years at the lower idle current/higher plate voltage condition. Nothing to worry about. And higher (than spec.) idle current is worse than excess plate voltage, which is why we don't adjust bias to manipulate the plate voltage.

We don't have a schematic of the 220V export version of the amp. Some power transformers have a choice between a 220V or 240V tap. If it does, and it's set to 220V, changing it to the 240V tap will bring all the voltages down. Are there any unused wires on the power transformer?

[Helmholtz]

If heater voltage is close to 6.3V, everything should be fine.

[Sea Chief]

Change in plate voltage is a byproduct of idle current. The more current the tubes conduct, the more 'work' the power transformer is doing. That makes the voltage sag.
Like a garden hose pressure will drop as you increase the water flow.
When you had .36V across the cathode resistor, that worked out to 18mA per tube, lower current than what you have now. So the voltage would have been higher. You can't calculate what it would have been, just set it back to that setting and measure again at the standby switch.
Now consider that the amp will have been working fine for many years at the lower idle current/higher plate voltage condition. Nothing to worry about. And higher (than spec.) idle current is worse than excess plate voltage, which is why we don't adjust bias to manipulate the plate voltage.

We don't have a schematic of the 220V export version of the amp. Some power transformers have a choice between a 220V or 240V tap. If it does, and it's set to 220V, changing it to the 240V tap will bring all the voltages down. Are there any unused wires on the power transformer?

Hi g1 thanks for this. I recall you did explain similarly before, I just wanted to be sure ( measuring so close to 800v !).

Ok dumbass Q: would be safer to leave as it is now... or tweak back to how it was?

I know your analogy is needed for my hopeless brain like mine, & appreciated all the same.. but.. "Like a garden hose pressure will drop as you increase the water flow".. actually I don't get it. This must seem absurd to you, gnashing of teeth etc.

But if I increase the flow by opening the tap up to force more water through... I can't consider anything other than the pressure to -increase-. I'm doing my thought experiments, just like Einstein papers everywhere here etc, & nope.. I just cannot think of it decreasing in any scenario if I turn up the tap.

But nevermind! Here is a more intelligent Q (I promise..): if the 1976 Music Man service sheet says ' just measure 0.5v across the 10r cathode resistor' as a way of setting the bias, would this possibly be different NOW, IF its known my line voltage is -higher- than back then in 1976, by perhaps 14v ?

[Sea Chief]

If heater voltage is close to 6.3V, everything should be fine.

Hi HH. Forgive me but am I measuring from any heater pin.. to ground, or across pins 2,7?

I think there is difference depending on whether a centre tap is present.. is that correct?

thanks SC